for dseries (see 48115cb8, which does not work with Octave) or a
method (see ef923e41, which does not work with all supported Matlab's
versions) to initialize the dseries class.

Takes into account the change for the dseries initialization routine.

To initialize the toolbox:

 - Add the `src` folder in the path
 - run `dseries().initialize()`

This is an Octave compatibily workaround, Octrave cannot handle
simultaneously +folder (namespace) and @folder (class).

Using the old HTTP URL makes test fail.

For a zero, it returns -Inf.
For a negative number, it returns NaN.

This is not exactly the same behavior as MATLAB's log function, because we want
to enforce type stability (i.e. never return a complex).

'A', 'Q', '1900Q', 'Q1', 'M', '1900M', 'M1','W', '1900W' and 'W1'
should not be interpreted as dates.

All dseries syntaxes now work under Octave, except ts{'variable'}(range) = value.

On MATLAB, importdata() is better at automatically guessing the delimiter.
Enforce tab delimiter to make it work on Octave.

Closes #39

As a consequence, no need to download the binary on the CI runners, since it is
already there.

It is possible to rename the dseries class. For instance, the
following is a valid syntax:

>> tseries = dseries();
>> tseries(ones(3,2))

ans is a dseries object:

   | Variable_1 | Variable_2
1Y | 1          | 1
2Y | 1          | 1
3Y | 1          | 1

It is also allowed to define a default starting period for the data passed
to the renamed dseries class/object:

>> tseries = dseries('1938M11')
>> tseries(ones(83,2))

ans is a dseries object:

        | Variable_1 | Variable_2
1938M11 | 1          | 1
1938M12 | 1          | 1
1939M1  | 1          | 1
1939M2  | 1          | 1
1939M3  | 1          | 1
1939M4  | 1          | 1
1939M5  | 1          | 1
1939M6  | 1          | 1
1939M7  | 1          | 1
1939M8  | 1          | 1
        |            |
1944M11 | 1          | 1
1944M12 | 1          | 1
1945M1  | 1          | 1
1945M2  | 1          | 1
1945M3  | 1          | 1
1945M4  | 1          | 1
1945M5  | 1          | 1
1945M6  | 1          | 1
1945M7  | 1          | 1
1945M8  | 1          | 1
1945M9  | 1          | 1